The present invention relates to a mobile telephone capable of calling using two different calling systems such as a personal digital cellular system (PDC) employed in a cellular phone and a personal handyphone system (PHS) in a cordless telephone.
Description of the Related Art
Conventional mobile telephones have been widely used as information terminals capable of talking during moving. In such mobile telephones, two different calling systems, the personal digital cellular system (PDC) (called "a large scale cell system" hereinafter) used in a cellular phone and the personal handyphone system (PHS) (called "a small scale cell system" hereinafter) employed in a cordless phone have been widely used.
In the PDC, a user can talk during a high speed moving. However, a large consumption current is required and a calling charge is high. In the PHS, a telephone call cannot be done during a high speed moving. But, a consumption current is small and a calling charge is small. These two different systems have both merits and demerits.
In practice, when a telephone is connected in the PHS, as often occurring in moving by an automobile, while a high speed moving and a low speed moving are repeated, a user can talk when the automobile moves at a low speed, for example, waiting at a traffic signal. However, when the automobile moves again and the telephone is suddenly disconnected at a hand-off disabling speed. Such a trouble takes place on a train moving again after stopping at a station in the same manner.
In the PHS, during the telephone call, the call can be disconnected as the terminal moves at the hand-off disabling speed. In order to prevent this trouble, the PDC can be used. However, as described above, the PDC requires the large consumption current and the high calling charge, and the full time use is expected to be avoided.
In order to solve this problem, other mobile telephones capable of using the PDC and the PHS have been proposed. In these mobile telephones, the two calling systems can be switched depending on the using situation so as to make the best use of both the merits.
One mobile telephone of this kind is disclosed in Japanese Patent Laid-Open Publication No. 6-224828, as entitled "Micro Cell/Macro Cell Integrated Mobile Communication System", Akira YAMAGUCHI et al. In this telephone in which a plurality of calling systems are integrated in one terminal, a moving speed of the terminal is estimated from a receiving level change, and the systems are switched depending on the moving speed even during the talking, with the result of applying both the merits.
Another mobile telephone of this kind is disclosed in Japanese Patent Laid-Open Publication No. 59-15573, as entitled "Telephone To Be Mounted To Mobile Body," Akiho SASAKI et al. In this telephone, a moving speed of a terminal is detected, and the calling systems are switched on the basis of the moving speed, utilizing both the merits.
In the first conventional telephone, it is necessary to measure the receiving levels of a plurality of radio control channels transmitted from base stations and their change with the elapse of time. However, now, there is no hope of realizing the hand-off between the two different systems such as the PDC and the PHS of different enterprising parties. In this case, the moving speed can not be measured. Moreover, the receiving level varies largely depending on states of propagation paths, and the precise moving speed of the terminal cannot be estimated.
In the second conventional telephone, a method of linking the motion between a speedometer of a mobile body and a terminal is proposed in order to detect the moving speed of the terminal. However, in this method, when the terminal is used on a train or the like, the detection of the moving speed cannot be carried out.